.parent {
  width: 500px;
  background-color: pink;
  padding: 8px;
}

.son {
  width: 100%;
  background-color: skyblue;
  padding: 8px;
  box-sizing: border-box;
}

.son-son {
  width: 100%;
  background-color: purple;
  padding: 8px;
  box-sizing: border-box;
}